1. Field of the Invention
The present invention relates generally to a limb load monitoring system, and more particularly to a lower limb load monitoring device for measuring or detecting the amount of force applied to, or weight borne by, a lower limb of the body (either natural or prosthetic) and providing a signal to the user when a predetermined threshold level is exceeded.
2. Description of the Prior Art
Numerous situations exist where it is important to limit the load or force which is applied to or borne by a lower natural or prosthetic limb of the body during standing, walking, stepping, running or jumping activities or during rehabilitation therapy. Situations also exist where it is important that the lower limb be exposed to a certain load or force, particularly during rehabilitation therapy. In both situations it is important to monitor such load or force and to provide a signal to the user when such force is exceeded or met. Examples include post-surgery or injury rehabilitation of hips, knees, ankles or any other portion of the body which is affected by force applied to or borne by at least one of the user's legs or any other situation in which monitoring of the weight on a lower limb during standing, walking, jumping or other activities is desired. Because of the clear economic benefits, including the potential of substituting outpatient care for clinical rehabilitation and the speeding or other enhancement of the rehabilitation process, the health care industry is exhibiting increased interest in improved devices which detect and monitor the force applied to the lower limbs.
Various patents currently exist which reflect the state of the art. One device is illustrated in U.S. Pat. No. 3,702,999 issued to Gradisar. In this device, a force sensitive means is positioned in two predetermined locations in the shoe of the user via a foot pad assembly: one beneath the heel and the other beneath the ball of the user's foot. Each of the force sensing means in this patent comprise a pair of electrical conductors which are spaced by a resilient dielectric member. Each of the force sensing means includes a set screw threadedly received by one of the conductors and extending toward the other. As a compressive force is applied to the pair of conductors, the resilient dielectric compresses causing the set screw to engage the other conductor. This closes the circuit and results in the generation of a signal. If the user desires the amount of force to be varied, the adjustment of the set screw is varied. Although this device conceptually provides a signalling device when a predetermined load is exceeded, the force sensing means themselves require a relatively thick profile since its operation requires vertical movement between two conductive plates with springloads between them. This creates a bulky orthopedic monitor which impedes the patient's normal walking style. Additionally, because of the mechanical nature of the pair of conductors, and the manner in which the set points are achieved, the patient, in some cases, must don and doff the device several times before achieving the correct calibrated set point. Also, variations in performance are inherent due to material wearing and aging. Still further, there is no means to combine the forces or loads of the two sensing means.
A second device is described in U.S. Pat. No. 4,745,930 issued to Confer. This device, however, is not a weight monitoring device, but instead is a force sensing insole used in association with an electro-goniometer for analyzing the gait of a patient. The device includes a multi-layer structure and a plurality of switches which sequentially close and open as the weight of the user is applied to the insole.
A further device described in U.S. Pat. No. 4,647,918 issued to Goforth relates a multi-event notification system for monitoring preselected critical pressure points on the feet of the user. This device discloses a plurality of battery powered pressure transducers in foot sensor pads for measuring the pressure at a number of points as a function of time. The information from such sensors is important for persons who have been diagnosed as having diminished sensation in the feet such as from diabetes mellitus. There is no disclosure in this patent of a device for warning the user of excessive instantaneous application of pressure or weight and accordingly it is not suitable for application as a lower limb load monitor.
Still other devices are described in U.S. Pat. No. 3,791,375 issued to Pfeiffer and U.S. Pat. No. 3,974,491 issued to Sipe. Although both relate generally to devices for signalling when excessive load is being borne by the lower limb, both devices rely on the detection of pressure created in a fluid containing load cell of the like. Thus, both are susceptible to failure from puncture to the fluid bladders. Further, contamination or degradation of the fluid can cause drift of sensor calibration or failure of the sensor in operation. Still further, the mechanical nature of the fluid transfer and the size of the tube required to transfer load information to a monitoring device necessarily requires a relatively bulky structure.
Accordingly, there is a need in the art for a lightweight, compact lower limb load monitor device which utilizes a relatively thin force sensing means for sensing both total load and load distribution.